International standard ISO11609 of 1995 contains the following definitions:
Dentifrice: any substance or combination of substances specially prepared for the public for cleaning the accessible surfaces of teeth.
Toothpaste: any semi-solid dentifrice preparation presented in the form of a paste, cream or gel.
This invention is concerned with toothpaste.
Toothpaste invariably contains an abrasive powder for cleaning purposes. The cleaning and abrasive properties of the toothpaste depend on the concentration of the powder, on the Moh""s hardness of the abrasive, and on the shape and particle size and size distribution of the powder. The cleaning effect of toothpaste concerns its effectiveness in removing adventitious stains and other deposits from the surface of teeth and other parts of oral cavity. The abrasive effect or abrasivity concerns undesired removal of surface components of teeth including enamel and dentine, and undesired damage in the oral cavity. A toothpaste having a high cleaning effect generally has a rather high abrasivity, and vice versa. ISO11609 provides methods for testing abrasivity and sets limits on dentifrices marketed in Europe. An earlier British standard BS5136 of 1981 also sets limits on abrasivity by comparison with a standard reference toothpaste, which is also used as a reference in ISO11609, and which has the formulation:
Precipitated calcium carbonate 40% w/w.
Glycol 25% w/w
Sodium carboxymethylcellulose 1.40% w/w
Dodecyl sodium sulphate 1.00% w/w
Sodium silicate (80xc2x0 TW of approximately pH 7) 0.05% w/w
Saccharin sodium 0.15% w/w
Formalin (40% (m/m) formaldehyde) 0.10 w/w
Peppermint flavouring 0.80% w/w
Water 33.05% w/w
Abrasive powders used or contemplated for use in toothpaste formulations include silicas, including gels and precipitates, sodium bicarbonate, calcium and magnesium carbonates, calcium phosphates, alumina and hydrates thereof, aluminosilicates, aluminium and magnesium silicates, and thermosetting urea-formaldehyde and other plastics materials. There is a need in the industry for an abrasive for inclusion in toothpaste formulations, which provides good cleaning properties, in particular stain removal, while showing relatively low abrasivity. In particular, there is a need for an abrasive which can be incorporated in a concentration large enough to provide an excellent cleaning effect in a toothpaste with low abrasivity which nevertheless complies with the abrasivity requirements of the above-stated standard specifications. This invention addresses that need.
U.S. Pat. No. 3,957,968 teaches the use of flat flakes of xcex1-aluminium oxide in toothpaste. The flakes have a median particle size of 2 to 7 xcexcm. U.S. Pat. No. 4,060,599 discloses the use of a finer aluminium oxide (median 1 to 2 xcexcm) and specifically uses Reynolds RC152DBM which has a median size of around 1.7 xcexcm.
U.S. Pat. No. 4,632,826 teaches the use of a weakly calcined alumina polishing agent. This polishing agent consists of 10% to 50% by weight xcex3 aluminium oxide and 50% to 90% by weight xcex1 aluminium oxide.
GB-A-2155333 teaches the use of calcium hydrogenphosphate anhydride and an aluminium oxide having an average particle size of 0.5 xcexcm to 10 xcexcm. The alumina has a high xcex1-content, as measured by X-ray diffraction.
WO-A-95/33441 teaches the use of cationically charged colloids of a metal compound. The colloid has a particle size of 0.001 xcexcm to 0.2 xcexcm.
GB-A-2037162 and GB-A-2009596 disclose the use of hydrated aluminas in dentifrice products.
The phrase xe2x80x9caluminaxe2x80x9d is sometimes loosely used to cover a number of aluminium oxide, oxide hydroxide and trihydroxide compounds. The correct designations and some crystalline phases are shown below:
This invention is concerned with alumina that is essentially Corundum. Corundum is produced by calcination of aluminium trihydroxides and oxide hydroxides. Depending on the form of aluminium trihydroxide and oxide hydroxide starting material, a number of forms of alumina (often described as activated aluminas) are produced before corundum is formed. Another form of alumina chemical is aluminium hydroxide gel, which is often formed by the neutralisation of an aluminium salt solution.
The invention provides a toothpaste comprising as an abrasive an alumina in the form of particles having d10 below 3.5 xcexcm, d50 below 1.0 xcexcm, and specific surface area below 6 m2/g.
According to a further aspect of the present invention, there is provided the use of alumina as an abrasive in toothpaste, wherein the alumina is in the form of particles having d10 below 3.5 xcexcm, d50 below 1.0 xcexcm, and a specific surface area below 6 m2/g.
According to a further aspect of the present invention, there is provided the use of alumina as a whitening agent in toothpaste, wherein the alumina is in the form of particles having d10 below 3.5 xcexcm, d50 below 1.0 xcexcm, and a specific surface area below 6 m2/g. In this way the alumina may serve the purpose of replacing titania or the like in conventional products, thus resulting in a cost saving.
The surface area is measured by the following method. A sample of alumina of sufficient weight to give an estimated surface area of about 0.5 to 25 m2 is degassed in a Micrometrics Desorb 2300B at about 150xc2x0 C. until a stable reading is obtained. The sample is then transferred to a Micrometrics Flowsorb II 2300, cooled and immersed in a mixture of 30% N and 70% He gas. The total amount of N absorbed is measured from the change in thermal conductivity of the gas mixture preferably during desorption as the temperature is raised again to room temperature. Surface area per gram is then calculated from the total gas absorbed and the weight of the sample.
The particle size is measured as follows. Particle size distribution of a sample of alumina is measured in a Sedigraph 5100 instrument supplied by Micrometrics Products.
The abrasive is preferably an anhydrous alumina, generally a calcined alumina or alternatively a tabular or fused alumina. Calcination is effected at a temperature of at least 900xc2x0 C. Higher calcination temperatures result in harder products. The alumina used in this invention is preferably fairly hard, such as may be obtained by calcination at above 1000xc2x0 C. The alumina in this invention is an xcex1-alumina. Preferably, the xcex1 content is greater than 90%, preferably greater than 93%, even more preferably greater than 95% as measured by X-ray diffraction. In a preferred embodiment, the xcex3-content is less than 1%.
The abrasive is used in the form of particles having d10 below 3.5 xcexcm, preferably below 2.5 xcexcm. (d10, d50 and d90 are used in conventional manner to indicate that 10, 50 or 90 wt % of the product has a particle size above the stated value). Preferably, the alumina abrasive is a sub-micron product, having d50 of 0.1-1.0 xcexcm. Preferably the alumina abrasive has a relatively narrow particle size distribution, e.g. with d10 being no greater than four times d50.
The alumina abrasive has a specific surface area below 6 m2g, preferably in the range of 4.5-5.0 m2/g. The specific surface area is related to the aforesaid parameters of hardness (harder products have lower specific surface areas) and particle size (more finely divided products have larger specific surface areas).
Alumina products of the kind described are readily available commercially, for they are produced in substantial quantities mainly for use in refractories and ceramics. Grinding may conveniently be effected by fluid energy or vibratory milling (micronising) or preferably by ball milling.
When an abrasive powder has excellent stain removal properties in one toothpaste, it is generally the case that it will be found to have excellent stain removal properties in others. Although the abrasivity of a toothpaste does depend to a significant extent on the whole formulation, and not merely on the nature and concentration of the abrasive present in it, nevertheless an abrasive which shows high or low abrasivity in one toothpaste formulation may generally be expected to show correspondingly high or low abrasivity in others.
The alumina abrasive is preferably present in the toothpaste at a concentration of 1-15% w/w, preferably higher than 2%, even more preferably higher than 3%, e.g. 3-12% w/w. Toothpastes include a wide variety of components in a wide variety of concentrations. The alumina should be compatible with other ingredients. The following list is intended to be exemplary rather than definitive or restrictive.
Toothpastes are generally water-based.
A binder or thickener is generally present. Examples of suitable materials are carboxyvinyl polymers, carrageenan, hydroxyethyl cellulose and water soluble salts of cellulose ethers such as sodium carboxymethylcellulose and sodium carboxymethyl hydroxyethyl cellulose. Natural gums and colloidal silica or silicate materials can also be used. Binders or thickeners are generally present in an amount from about 0.15 to about 5.0% w/w of the total composition.
A humectant is also generally used to keep the formulation from hardening on exposure to air. Examples of suitable humectants are glycerine, sorbitol, xylitol, polyethylene glycols and propylene glycol. Humectants are generally present in an amount from about 10% to about 70% w/w of the weight of the composition.
One or more particulate materials, regarded as abrasives or abrasive polishers or fillers, are also present. The alumina abrasive described above is one such, but others may also be present as noted above. Examples are:
Silicas, including gels and precipitates, sodium bicarbonate, calcium and magnesium carbonates, dicalcium phosphate dihydrate, alumina and hydrates thereof, aluminosilicates, aluminium and magnesium silicates, and thermosetting urea-formaldehyde and other plastics materials. It is preferred that the alumina is not present in combination with calcium hydrogenphosphate anhydride.
Abrasives are generally present at a level of about 10% to about 70%, preferably from about 15% to about 25%, w/w of the formulation.
A source of fluoride ion is preferably provided. Regulatory authorities in various countries may stipulate a maximum and/or a minimum total fluoride ion concentration.
Other components which may also be included in toothpaste formulations in accordance with conventional practice, include sweeteners, flavours, colours, peroxides or other bleaching agents, anti-calculus agents, anti-plaque agents, anti-bacterial agents, preservatives and effervescence generators.
Toothpaste may be manufactured by conventional techniques. The alumina abrasives with which this invention is concerned are rather easy to incorporate for, unlike some other abrasive powders, they are generally not prone to lumping.
Here is an example of a toothpaste formulation:
Dicalcium phosphate 40% w/w
Alumina 10%
Sorbitol 25%
Polyethylene glycol 2%
Carboxymethylcellulose 1.1%
Sodium Saccharin 0.2%
Sodium Lauryl Sulphate 1.5%
Benzoic Acid 0.15%
Sodium Benzoate 0.2%
Water 19.5%
This formulation based on dicalcium phosphate polishing agent/filler, has been used to test the properties of various alumina abrasives. In the experimental section below, a different formulation based on a silica polishing agent/filler was used. Although the results obtained with the two formulations were different in absolute terms, they were generally the same in relative terms, i.e. any particular alumina abrasive generally had the same or a similar effect on the properties of both formulations. This justifies the assertion that an abrasive showing valuable properties in one toothpaste may reasonably be expected to show corresponding valuable properties in others.
The alumina abrasives used in the experimental section below are either commercial products or development products (designated BAX) available from Alcan Chemicals Limited, and are here identified by means of their trade names. This invention is based on the discovery that the development product BAX842 has unexpectedly and outstandingly good properties.